This invention relates to antenna arrays, and more particularly, to antenna arrays used for uplink and downlink communication in cellular and other wireless communication systems.
Every antenna has a directionally-dependent response function, which is often referred to as the xe2x80x9cradiation patternxe2x80x9d in transmission, and as the xe2x80x9csensitivity patternxe2x80x9d in reception. It has long been known that when multiple antenna elements are assembled in an antenna array, the shape of this response function can be tailored by applying suitable complex-valued weights (which combine specified phase delays with specified attenuation coefficients) to the respective elements. One particular known advantage of such arrays is that by actively changing the weight coefficients, it is possible to maximize the transmitted or received power in a specified direction. An antenna array effective for that purpose is one example of an adaptive array.
In the field of cellular communications, it is an ideal, but generally unreachable, goal for each base station to transmit power only to mobile stations within a designated reception area, and to be sensitive to transmissions only from those mobile stations. One proposed approach to this goal is for the base station to transmit and receive using an adaptive array that seeks to maximize its response function at the mobile stations within its reception area.
For example, during uplink transmission from a given mobile station, it is possible for each element of the array to measure a respective propagation coefficient characterizing the physical channel between itself and that mobile station. This coefficient is desirably sampled over time. In TDMA systems, for example, the propagation coefficient from each mobile station, in turn, can be sampled in each of the, e.g., 162 symbol periods that occupy that mobile station""s time slot. The time-averaged samples can be assembled into a covariance matrix for each mobile station. Recently, a technique has been described for obtaining, from each of these covariance matrices, a set of weight coefficients that will tend to concentrate the response function in the direction of the pertinent mobile station. This technique is described, e.g., in G. G. Raleigh et al., xe2x80x9cAdaptive Antenna Transmission for Frequency Duplex Digital Wireless Communication,xe2x80x9d Proc. Int. Conf. Comm., Montreal, Canada (June 1997).
However, there are certain obstacles to putting this scheme into successful practice. Measures are necessary to prevent interference between the uplink and downlink signals. The most common such measure, at least in TDMA systems, is referred to as frequency-division duplex transmission (FDD). In FDD there is a shift, typically 5%-10%, between the uplink and downlink carrier frequencies (or, equivalently, between the corresponding wavelengths). This shift is sufficient for the receivers at the base station and mobile stations to readily distinguish between the uplink and downlink signals. Thus, it is possible for uplink and downlink transmissions to overlap in time. (Although there are also time-division duplex systems, in which such overlap is forbidden, the use of these systems is less favored.)
The response function of an antenna array is dependent upon the frequency of transmission or reception. Therefore, in a FDD system, the uplink sensitivity pattern is different from the downlink radiation pattern. A set of weight coefficients derived adaptively on the uplink to provide a certain directionality will not, in general, provide the same directionality on the downlink.
There are known formulas for deriving a new set of coefficients, effective for the downlink wavelength, from the uplink weight coefficients. However, these formulas generally require the direction to the targeted mobile station to be known with more precision than is available from the covariance matrices alone. The operations required to provide such directional information are complex and time-consuming, and for that reason are disfavored.
Because of the obstacles described above, it is desirable to provide the base station with a system of antennas that can obtain statistical information concerning the uplink propagation coefficients, and then obtain weights from this statistical information for use on the downlink.
One proposed approach is for such a system to consist of two distinct antenna arrays, one for the uplink, and the other for the downlink. It is known that two antenna arrays, operating at distinct frequencies (and thus, distinct wavelengths), will have identical response functions if they are identical except for scale, and if their relative scale factor is equal to the ratio of the respective wavelengths. That is, let the spacing between each pair of elements of array 2 be r times the spacing of the corresponding elements of array 1. Let array 1 operate at wavelength xcex1, and let array 2 operate at wavelength xcex2.
Then the arrays will have the same response function if   r  =                    λ        2                    λ        1              .  
Thus, one wavelength can be taken as the uplink wavelength, and the other as the downlink wavelength. Distinct uplink and downlink antenna arrays can be installed, geometrically similar but having relative scales determined by the wavelength ratio.
However, such an approach has certain disadvantages. One disadvantage is the expense of a second antenna installation. Another disadvantage relates to the relative siting of the respective antenna arrays. If the arrays are sited too close to each other, they will suffer undesirable mutual coupling effects. If, on the other hand, they are sited too far from each other, the weights selected from the uplink measurements may not function properly on the downlink, at least for relatively close mobile stations.
Therefore, there continues to be a need, in the cellular wireless field as well as other fields in which uplink and downlink antenna arrays can utilize directionality, for a practical system of antennas whose response function is insensitive to wavelength shifts.
We have invented an antenna array having a response function that is insensitive to shifts between pairs of wavelengths that stand in a specified ratio.
In a broad aspect, our invention involves a system for sending wireless communication signals on at least one downlink wavelength and receiving wireless communication signals on at least one uplink wavelength, wherein there is a ratio r equal to the larger divided by the smaller of these wavelengths. The system comprises a receiver operative to receive signals imposed on a carrier having the uplink wavelength, a transmitter operative to transmit signals imposed on a carrier having the downlink wavelength, and an array of independent antenna elements. (By xe2x80x9cindependentxe2x80x9d is meant that these elements can be separately driven, or separately used for the reception of radiofrequency signals.)
The array comprises a first and a second sub-array. One sub-array is electrically coupled to the transmitter, such that transmitted signals can be radiated from it, and the other sub-array is electrically coupled to the receiver, such that signals to be received can be extracted from it.
The sub-arrays are geometrically similar to each other; i.e., each antenna element of one array has a counterpart in the other sub-array, and corresponding inter-element spacings stand in a constant ratio. Thus, the constant ratio is a scale factor that relates the dimensions of one sub-array to the dimensions of the other. This scale factor is equal to the wavelength ratio r.
Significantly, the sub-arrays have at least one common antenna element.
In specific embodiments, the elements of the full array are arranged in a one-, two- or three-dimensional array having, respectively, one, two, or three lattice directions. At least three elements, and not more than a respective maximum number of elements, are arranged from first to last along each lattice direction of the array. Along each lattice direction, the elements are spaced with a constant ratio between successive spacings.
The array comprises first and second sub-arrays. Along each lattice direction, there is at least one row in which the first Mxe2x88x921 elements belong to the first sub-array, and the last Mxe2x88x921 elements belong to the second sub-array, where M is the respective maximum number of elements along that lattice direction.